6502 ASM FTW
It's true. I hand-coded and hand-assembled many KB of programs for my 6502 KIM-1 back in the day. I knew the machine codes in hex for pretty much the entire CPU. To this day I can't see "A9" anywhere without thinking "LDA #" (load accumulator immediate).
God, and to think I could be using those brain cells for something useful now...
6809